The present invention pertains to the production of oxygen by an integrated air separation/gas turbine process. In a gas turbine combined cycle power generation process, ambient air is adiabatically compressed and combusted with a fuel gas in a combustor. The combustion product is work expanded to slightly above atmospheric pressure, and the generated work is utilized to drive the compressor and usually an electric generator. The expander exhaust, which contains valuable high level heat, is introduced into a heat recovery steam generation (HRSG) system to recover high level heat as steam. The steam is expanded through a steam turbine which drives an additional electric generator.
A portion of the compressed air may be withdrawn from the gas turbine compressor for a variety of uses, including feed air (typically called "extracted air") to an air separation unit, as cooling for the turbine itself, or for other pressurized air requirements within the facility. The extracted air contains valuable heat that can be recovered at discrete temperature levels by vaporization and expansion of working fluids or by recovering sensible heat for transfer into another process fluid. An inert gas stream, such as steam or nitrogen, may be injected into the combustor for reduction of nitrogen oxides in the exhaust gas and for additional gas motive flow to the expander.
An air separation unit (ASU) for the production of oxygen may be integrated with a gas turbine combined cycle, and the oxygen used for example in a coal gasification system to provide fuel for the gas turbine combustor. A nitrogen-rich product, which is considered a waste stream if nitrogen is not required in the process, also is produced by the air separation system. Some or all of the ASU feed air typically is extracted from the gas turbine compressor at an elevated temperature. The expansion turbine exhaust can be introduced into an HRSG as described earlier for additional heat recovery via a steam turbine system for additional power generation.
The utilization of the nitrogen-rich waste stream is an important factor in the overall efficiency of integrated air separation/gas turbine systems, and several methods have been described in the prior art for such utilization. In one well-known and widely-used method, energy is recovered from the hot extracted air to provide cooled air for the ASU feed by cooling the extracted air against a compressed nitrogen-rich waste stream, which may be further heated by heat exchange with hot process streams. The resulting heated and compressed nitrogen-rich waste stream is injected into the gas turbine combustor, or alternatively into the gas turbine expander, to recover energy from the stream and thereby reduce the fuel required for combustion. In addition, the inert gas flow to the combustor reduces nitrogen oxide formation and increases the motive flow into and power output from the expansion turbine. This method of utilizing the nitrogenrich waste stream is described in representative U.S. Pat. Nos. 4,250,704, 4,697,415, 5,081,845, and 5,406,786 and in European Pat. Nos. Application EP 0 773 415 A2.
Another method of utilizing the nitrogen-rich waste stream in an integrated air separation/gas turbine system is described in U.S. Pat. Nos. 3,731,495, 4,019,314, and 5,406,786 wherein this stream is optionally heated and introduced directly into the gas turbine expander without prior compression.
Alternatively, the nitrogen-rich waste stream can be expanded in a separate expansion turbine which drives an electric generator or a process stream compressor as described in U.S. Pat. Nos. 4,019,314 and 5,410,869. The exhaust from this separate expansion turbine, if at a sufficiently high pressure, may be introduced into the gas turbine expander.
U.S. Pat. Nos. 5,388,395 describes an integrated air separation/gas turbine system in which the nitrogen-rich waste stream is cooled by work expansion to drive an electric generator, and the expanded cooled nitrogen stream is introduced into the gas turbine air compressor inlet to cool the total inlet stream. This improves the compressor efficiency and thus the overall gas turbine efficiency. Alternatively, if the nitrogen-rich waste stream is at a low pressure, the stream is chilled and humidified by direct contact with cold water and introduced into the gas turbine compressor.
An alternative use for the nitrogen-rich waste stream in an integrated air separation/gas turbine system is described in U.S. Pat. Nos. 4,729,217 wherein a portion of this stream is combined with fuel gas and fired in a waste heat recovery boiler with the gas turbine exhaust. Steam from the recovery boiler is expanded to generate electric power.
Great Britain Patent Specification 1 455 960 describes an air separation unit integrated with a steam generation system in which a nitrogen-rich waste stream is heated by indirect heat exchange with hot compressed air from the air separation unit main air compressor, the heated nitrogen-rich stream is further heated indirectly in a fired heater, and the final hot nitrogen-rich stream is work expanded in a dedicated nitrogen expansion turbine. The work generated by this expansion turbine drives the air separation unit main air compressor. The nitrogen expansion turbine exhaust and the combustion gases from the fired heater are introduced separately into a fired steam generator to raise steam, a portion of which may be expanded in a steam turbine to drive the air separation unit main air compressor. Cooled nitrogen is withdrawn from the steam generator and may be used elsewhere if desired. Optionally, the combustion gases from the fired heater are expanded in a turbine which drives a compressor to provide combustion air to a separate fired heater which heats the nitrogen-rich stream prior to expansion. In another option, the nitrogen expansion turbine exhaust and the combustion gases from the fired heater are combined and introduced into the economizer and air preheater sections of the fired steam generator.
Thus the prior art teaches a number of useful methods for the recovery of heat from extracted air in an integrated gas turbine/air separation system. The most efficient methods require additional equipment such as heat exchangers, steam generators, fired heaters, steam turbines, compressors, electric generators, and the like. A high-efficiency integrated gas turbine/air separation system which utilizes this additional equipment will have a higher capital cost and greater operating complexity than a simpler, less-efficient system.
In certain industrial applications of integrated gas turbine/lair separation systems, high efficiency may not be required, while low capital cost and minimum operating complexity may be important. One such application is the production of oxygen for the conversion of natural gas to liquid fuels in remote locations far from industrialized areas. In this application, low-cost fuel is readily available and moderate energy efficiency is acceptable, but capital equipment costs are high and operating complexity is undesirable. In addition, electric power usually is not available at such locations, and the export of electric power or steam outside of the integrated gas turbine/air separation system and associated process area generally is not feasible. A limited amount of electricity and steam can be generated for use within the integrated gas turbine/air separation system and associated process area.
The conversion of natural gas to liquid fuels in remote locations thus requires an integrated gas turbine/air separation system designed to achieve a balance among energy efficiency, capital cost, and process simplicity. The invention described in the specification below and defined by the claims which follow addresses the need for simple, low-capital, and reliable oxygen production by an integrated gas turbine/air separation system specifically designed for operation at remote locations.